Gothmog was initially one of the Maiar who swore under the allegiance of Melkor. Prior to the Revolt of the Ñoldor, he was the High Captain of Angband and given the title Lord of the Balrogs.
During the Dagor-nuin-Giliath in YT 1498,Fëanor pressed to Angband with his company, and in that hour, the Balrogs were issued forth. Gothmog was among them, smiting the High King of the Ñoldor to the ground. The Sons of Fëanor arrived and carried the body of their father away, but Fëanor had passed. Gothmog reappeared as a general of Angband in several more major conflicts, including the Dagor Aglareb and the Nirnaeth Arnoediad. During the Nirnaeth, Gothmog engaged the High King of the Ñoldor, Fingon, in battle. Gothmog separated Fingon from the main host but was unable to smite Fingon, until another balrog appeared behind the Ñoldo "cast a throng of steel about him". This allowed Gothmog to slay Fingon and beat the body to dust. Thereafter, he captured Húrin, father of Túrin Turambar, and dragged him back to Angband.
In FA 510, Gothmog and the forces of Angband beseiged the Hidden City of Gondolin. They held the northern gates and were later confronted by Ecthelion of the Fountain, whom Gothmog dueled. Knocking Ecthelion's sword out of his hands, Gothmog prepared to strike at him but, as he raised his axe, Ecthelion ran forward at the Balrog lord, ramming his pointed helmet into the beast's chest and forcing both of them into the Fountain of the King. The waters quenched the flames of Gothmog that kept him alive and drowned Ecthelion, ending the Battle of Gondolin.